Prior U.S. Pat. No. 4,176,063 discloses a reverse-osmosis water purifier, which is commercially practical, and which does not use water after the pure water storage tank is filled. In contrast to such system, there are on the market systems which never shut off, even after the storage tank is filled, so that even a small home unit may waste thousands of gallons of water per month. A problem which exists relative to water-saving systems of the type shown in such U.S. Pat. No. 4,176,063, and in an earlier U.S. Pat. No. 4,021,343, is that after shut down of the system, the pure water standing on the pure water side of the reverse-osmosis membrane becomes contaminated with salts which perminate the membrane due to the osmotic pressure on the salt water side. Then, when the system starts up again, this contaminated "pure" water flows into the pure water storage bag to reduce substantially the purity of the water therein.